1. Field of the Invention
The object of the present invention is a seafloor—surface connection installation of at least one sub-marine hybrid tower pipeline installed at great depth.
The technical field of the invention is the field of manufacturing and installation of production vertical risers for sub-marine extraction of petroleum, gas or other soluble or fusible material or of a suspension of mineral material using an immersed well head for the development of production fields installed on the open sea, offshore. The main and immediate application of the invention being in the field of petroleum production.
A floating base comprises in general anchoring means for remaining in position regardless of the currents, winds and the swell. In general, it comprises also petroleum storage and processing means as well as offloading means to the oil -tank carriers, these latter appearing at regular intervals in order to carry out removal of production. In English these floating bases are called “Floating Production Storage Offloading”, for which the abbreviated form FPSO will be used throughout the following description.
By reason of the multiplicity of the lines existing on this type of installation, seafloor—surface hybrid type vertical riser type connections have been deployed in which the substantially vertical rigid pipelines called herein ‘vertical risers’, assure the connection between the sub-marine pipelines resting on the seafloor and rise along a riser up to a depth in proximity to the surface, at which depth the flexible pipelines assure the connection between the top of the tower and the floating base. The tower is thus equipped flotation mans in order to remain in a vertical position and the risers are connected at the foot of the tower to the sub-marine pipelines by flexible sleeves that absorb the angular movements of the tower. The assembly is collectively called a ‘hybrid tower’, because it combines two technologies: on the one hand a vertical part, the tower, in which the riser is comprised of rigid pipelines and on the other hand the top part of the riser comprised of concatenated flexible pipes that assure the connection to the floating base.
2. Discussion of Related Art
French patent FR 2 507 672 published on 17 Dec. 1982 entitled “Riser for Great Water Depths” is well-known and describes such a hybrid column.
The present invention relates more particularly to the known field of connection of the type comprising a hybrid vertical column anchored on the seafloor and comprised of a float disposed at the top of a vertical riser, the latter being connected by a pipeline, especially a flexible pipeline assuming by its own weight the form of a catenary from the top of the riser up to a floating base installed on the surface.
The interest in such a hybrid column resides in the possibility for the floating base to be able to leave its normal position by inducing a minimum of stress in the column as well as in the parts of the pipelines in the form of suspended catenaries, both on the seafloor and on the surface.
The patent filed on behalf of this applicant, WO 00/49267, is well known and discloses a column whose float is at a depth greater than the half-depth of the water and whose catenary connection to the surface craft is realized by means of thick-walled rigid pipelines. The column thus described requires at its base flexible link pipes enabling connecting the lower end of the vertical risers of said column to the sub-marine pipeline resting on the bottom in such a fashion as to absorb the movements resulting from dilations due to the temperature of the fluid being transported.
More particularly, in WO 00/049267, the anchoring system comprises a vertical tendon comprised of either a cable or a metal bar or even of a pipeline held at its upper end by a float. The lower end of the tendon is fixed to a foot resting on the seafloor. Said tendon comprises guide means disposed over its entire length through which said vertical risers pass. Said foot can be positioned simply on the seafloor and remain in place in virtue of its own weight or it can be anchored by means of piles or any other suitable device for maintaining it in place. In WO 00/42967 the lower end of the vertical riser can be connected to the end of a curved connection sleeve moveable between a high and a low position relative to said foot from which this connection sleeve is suspended and associated with a return means moving it into the high position in the absence of the riser. This mobility of the bent connection sleeve makes it possible to accommodate variations in the length of the riser under the effects of temperature and pressure. An abutment device is secured at the top of the vertical riser in suspension on the support guide installed on the top of the float.
Furthermore, the crude petroleum traveling over very long distances—several kilometers—it should be provided with an extreme insulation level in order on the one hand to minimize the increase in viscosity that would lead to a reduction of the hourly production of the wells and on the other hand to prevent blockage of the flow by deposition of paraffins or the formation of hydrates once the temperature decreases to around 30-40° C. These latter phenomena are even the more critical, in particular in West Africa, as the temperature of the sea bottom is of the order of 4° C. and the crude petroleum are of the paraffin type.
Numerous thermal insulation systems are known that make it possible to achieve the required performance level and to resist the pressure at the bottom of the sea which is of the order of 150 bar at 1,500 m depth. The “pipe-in-pipe” concepts can be cited inter alia that comprise a pipe carrying hot fluid installed within a protective external pipe, the space between the two pipes being either simply filled with a heat insulating material, whether or not confined under vacuum, or simple evacuated. Numerous other materials have been developed for assuring high performance insulation; certain of these being resistant to pressure, simply surrounding the hot pipeline and are in general confined to the inside of a flexible or rigid envelope under equipressure and whose main function is to maintain a substantially constant geometry over time.
All of these devices carrying a hot fluid within an insulated pipe exhibit, to different degrees, the phenomenon of differential dilation. In fact, the inside pipe, generally made of steel, is at a temperature that should be maintained as high as possible; for example 60 or 80° C., since the outer envelope, which is quite frequently also made of steel, is at the temperature of the seawater; that is, around 4° C. The forces acting on the connection elements between the inside pipe and the outer envelope are considerable and can reach several tens, even several hundreds of tons and the overall resulting elongation is of the order of 1 to 2 m in the case of insulated pipelines of 1,000 to 1,200 m in length.